1. Field of the Invention
The present invention relates to an inverter having the snubber and, particularly, to an inverter with an improved means for transferring stored energy in the snubber back to the d.c. power source.
2. Description of the Prior Art
FIG. 1 shows a half bridge construction of the conventional inverter circuit, in which the self turn-off switching devices are shown as arm elements. The arrangement includes positive and negative terminals P and N of a d.c. power source; arm elements 1U and 1X that are gate turn-off thyristors (will be termed simply GTO) receiving control signals on their gate terminals from a gate circuit (not shown) so that the GTOs turn on and off alternatively; a serial connection of reactors 2U and 2X connected between the GTO 1U and GTO 1X; feedback diodes 3U and 3X, one (3U) connected in anti-parallel configuration to the GTO 1U, the other (3X) connected in the same fashion to the GTO 1X; snubbers 4U and 4X, each made up of a snubber capacitor 41U (41X) and a snubber diode 42U (42X), connected in parallel to the GTOs 1U and 1X, respectively; a diode circuit 5 with its cathode side connected to the anode of the snubber diode 42U and with its anode side connected to the cathode of the snubber diode 42X; and a current transformer 6 having its primary winding connected in series to the diode circuit 5 and its secondary winding connected on terminals 6a and 6b to the a.c. input of a rectifying bridge 7 consisting of diodes 7a, 7b, 7c and 7d, while the d.c. output of the bridge 7 is connected between the terminals P and N of the d.c. power source.
Next, the operation of the above conventional inverter will be explained with reference to the timing charts of FIGS. 2a and 2b. In FIG. 2a, Tu and Tx represent the second conductive periods of the GTOs 1U and 1X, respectively, Iw is the load current of a load (inductive load in this example), Iu and Ix are the currents in the GTOs 1U and 1X, respectively, Iud and Ixd are the currents in the feedback diodes 3U and 3X, respectively, Vuc and Vxc are the voltages across the snubber capacitors 41U and 41X, respectively, and I.sub.D represents the current flowing through the diode circuit 5 and current transformer 6. FIG. 2b shows the waveform of the voltage and current of the current transformer 6 in a period immediately after the GTO 1U is turned off, with V.sub.CT showing the voltage across the primary winding of the current transformer 6.
At a time point t1 when the conductive GTO 1U is turned off, the current Iu which has been flowing in the GTO 1U is transferred to the snubber 4U, and the snubber capacitor 41U begins charging. At this time, charges in the capacitor 41X of the snubber 4X are discharged through the diode 5, current transformer 6, diode 42U, reactor 2U and a.c. output terminal U to the load (not shown). At time point t2, discharging completes, and the feedback diode 3X becomes conductive. During this period the secondary output of the current transformer 6 is fed back to the d.c. power source through the diodes 7a and 7d in the rectifying bridge 7, and a voltage proportional to the d.c. power source voltage is generated across the primary winding of the current transformer 6.
Subsequently, energy stored in the reactor 2U is fed back to the d.c. power source via the path including the reactor 2U, reactor 2X, snubber diode 42X, diode 5, current transformer 6, and snubber diode 42U, and the current flowing through the reactor 2U and current transformer 6 decreases. At this point t3 when the core of the current transformer 6 reaches a magnetic saturation, the current transformer 6 generates momentarily an excessive voltage in opposite polarity, but, the voltage between the terminals 6a and 6b is clamped to the d.c. power source voltage by the conduction of the diodes 7b and 7c. Although this reverse voltage serves to increase the current in a loop including the snubber diode 42U, reactor 2U, reactor 2X, snubber diode 42X and diode 5, the voltage drop in the loop is small and the current transformer 6 is reset to the voltage equal to this voltage drop. Also the current flowing through the reactor 2U and current transformer 6 decreases very slowly. At time point t4 when the polarity of the load current is reversed, the load current flows through the terminal U, reactor 2X and GTO 1X.
In the conventional snubber energy feedback means, as mentioned above, the reset operation for the current transformer 6 takes place in accordance with the voltage drop in the snubber diodes 42U and 42X, reactors 2U and 2X and diode circuit 5, resulting in a longer reset time needed. If the reset operation does not complete until the time point t1, the time point t3, at which the current transformer 6 is saturated comes earlier, resulting disadvantageously in the impairment of efficiency in transferring snubber energy back to the d.c. power source.